This invention relates to combined cycle power plants, and more particularly, it relates to a method for fuel gas heating to improve thermal efficiency of combined cycle power plants.
Combined cycle power plants and cogeneration facilities utilize gas turbines (GT(s)) as prime movers to generate power. These GT engines operate on the Brayton Cycle thermodynamic principle and typically have high exhaust flows and relatively high exhaust temperatures. These exhaust gases, when directed into a heat recovery boiler (typically referred to as a heat recovery steam generator (HRSG)), produce steam that can be used to generate more power. The produced steam can be directed to a steam turbine (ST) to produce additional power. In this manner, a GT produces work via the Brayton Cycle, and a ST produces power via the Rankine Cycle. Thus, the name xe2x80x9ccombined cyclexe2x80x9d is derived. In this arrangement, the GT Brayton Cycle is also referred to as the xe2x80x9ctopping cyclexe2x80x9d and the ST Rankine Cycle is referred to as the xe2x80x9cbottoming cycle,xe2x80x9d as the topping cycle produces the energy needed for the bottoming cycle to operate.
As technology progressed, the trend for the use of steam engines diminished and the use of steam turbines increased. One advantage of the steam turbine is its overall cycle efficiency when used in conjunction with a condenser. This approach allows the steam to expand significantly beyond normal atmospheric pressure and down to pressures that were only slightly above an absolute vacuum (0.5 to 2 pounds per square inch absolute (psia)). This allows the steam to expand further than in an atmospheric exhaust configuration, extracting more energy from a given mass of steam, thus producing more power and increasing overall steam cycle efficiency. This overall steam cycle, from a thermodynamic perspective, is referred to as the Rankine Cycle.
In many cogeneration and combination GT/ST power plants built today, combined cycle plants have come to mean power plants that utilize a Brayton Cycle as the topping cycle and a Rankine Cycle as the bottoming cycle. These plants utilize a gas turbine (GT) as the prime mover (Brayton Cycle machine), with a HRSG at the exhaust of the gas turbine to recover waste heat. The HRSG produces steam that is then supplied to a (ST) to produce more power. Due to the high temperatures of the working fluid in the GT and recovery of waste heat, the combined cycle plants are much more fuel efficient than the conventional steam plants.
Fuel gas heating in combined cycle power plants is typically performed to increase the thermal efficiency of the power plants. In one approach, hot water extracted from the exit of an IP economizer (i.e., the water entering an IP evaporator) of a heat recovery steam generator (HRSG) is used for fuel gas heating. In this approach, the maximum fuel gas heating temperature is limited by the temperature of the extracted water, which is typically lower than the saturation temperature of the IP evaporator. This approach limits fuel gas heating, thus limiting the efficiency of combined cycle power plants using IP water. Although higher fuel gas heating temperature improves the thermal efficiency of a power plant, a higher operating pressure of the IP evaporator has a detrimental effect on the steam cycle power output and the thermal efficiency of the power plant. Therefore, IP evaporator is typically operated at an optimum pressure in a combined cycle power plant, thus limiting the fuel gas heating temperature and the efficiency of the power plant.
In order to increase the temperature of the water available for fuel gas heating, water from high pressure economizers upstream of IP evaporator may be used. However, using high pressure water considerably increases the cost of fuel gas heating while presenting a reliability concern in the event of a failure. In one design, the available IP water temperature has limited fuel gas heating to 365xc2x0 F., in GE (F) class gas turbine combined cycle power plants. Thus, there is a need to improve the thermal cycle efficiency of combined cycle power plants overcoming the problems faced by prior systems.
Accordingly, the present invention provides a system and a method for recovering exhaust heat to further heat IP water for fuel gas heating to improve the thermal cycle efficiency of combined cycle power plants.
The system and method of the present invention increase the fuel gas heating temperature of combined cycle power plants, with IP water as the fuel gas heating medium, while maintaining the IP drum pressure at the steam cycle optimum. A fuel water heating section is provided in the HRSG at a location upstream of the intermediate pressure economizer (IP-EC) section of the HRSG and further between intermediate pressure evaporator tubes. A high pressure (HP) section and a low pressure (LP) section of the HRSG may also include one or more economizers arranged in various configurations. Also, each of the HP, IP, and LP sections may include superheaters. Fuel heating water is extracted from the IP-EC discharge and delivered to the fuel water heater section for further increasing the temperature of the fuel heating water prior to its delivery to the fuel gas heater. The higher fuel heating water thus allows the fuel to be heated to a higher temperature, thereby increasing the efficiency of the combined cycle power plant. The present invention overcomes the maximum heating temperature limits typically imposed by the IP drum operating pressure in the prior fuel gas heating methods.
In one aspect, a combined cycle power plant system, comprising a compressor; a combustor receiving air provided by the compressor; a gas turbine for expanding gas provided by the compressor; a heat recovery steam generator (HRSG) for receiving exhaust gases from the gas turbine, the HRSG having a low pressure (LP) section; a high pressure (HP) section, the HP section receiving exhaust gases from the gas turbine and located upstream of the LP section, each LP and HP sections having an evaporator; an intermediate pressure (IP) section located between the HP and the LP sections, the IP section comprising an economizer, first and second evaporators, and a water heater disposed between the first and second evaporators; and a fuel gas heater for receiving heated water from the water heater. The water heater may be located upstream of the economizer. The fuel-heating water extracted from the economizer is heated to a higher temperature in the water heater prior to delivery to the fuel gas heater. The water pressure in both the economizer and the water heater is maintained to prevent steaming of the fuel-heating water. The first evaporator is preferably located between the HP evaporator section and the water heater. The first evaporator may protect the fuel-heating water from steaming. The fuel-heating water is preferably heated to a temperature that is higher than the saturation temperature of the first and second evaporators.
In another aspect, the present invention describes a method of increasing the temperature of fuel-heating water in a combined cycle power plant, comprising flowing exhaust gas stream from a gas turbine through a heat recovery steam generator (HRSG), the HRSG having plural sections including an intermediate pressure (IP) section, a high pressure(HP) section, and a low pressure (LP) section; providing a water heater between first and second evaporators of the IP section, the second evaporator being located downstream of the first evaporator relative to the flow of the gas stream through the HRSG; flowing fuel-heating water from an economizer to the water heater, the economizer located downstream of the water heater; heating the fuel-heating water in the water heater; and delivering the heated water to a fuel gas heater.
In yet another aspect, a combined cycle power plant system, comprising: a gas turbine; a fuel gas heater; a heat recovery steam generator (HRSG) having plural sections including an intermediate pressure (IP) section with an evaporator having first and second evaporator sections, the HRSG in heat exchange relation with exhaust gases from the gas turbine; and a fuel-water heater disposed between the first and second evaporator sections for heating the fuel-heating water to a temperature higher than a saturation temperature of the evaporator.
In a further aspect, a method of heating fuel gas in combined cycle power plants, comprising: providing a heat recovery steam generator (HRSG) for receiving exhaust gases from a gas turbine; receiving water in the HRSG; providing first and second evaporators in an intermediate pressure section of a heat recovery steam generator; locating a water heater between the first and second evaporators for heating water; and delivering heated water from the water heater to a fuel gas heater.
In another aspect, a combined cycle power plant apparatus, comprising: a heat recovery system having a plurality of sections for receiving and recovering heat from a gas turbine exhaust; the heat recovery system comprising a water heater located in at least one section of the heat recovery system, the water heater capable of further heating fuel-heating water received from the one section of the heat recovery system; and a fuel gas heater receiving fuel-heating water further heated by the water heater.